(1) Field of the Invention
The present invention relates to a speech signal coding apparatus for encoding a speech signal to compress and transmit speech data, and a speech signal decoding apparatus for decoding the coded speech data to regenerate the speech signal.
(2) Description of the Related Art
In recent typical speech signal coding systems, a short term prediction coefficient is obtained by a short term prediction analysis in a short term prediction filter, a pitch prediction coefficient and a pitch period are obtained by a long-term prediction analysis in a long-term prediction filter, and a prediction residual signal is generated by inverse characteristic filters of the short and long-term prediction filters, and the above short term prediction coefficient, the pitch prediction coefficient, the pitch period, and the prediction residual signal are multiplexed and transmitted. Further, to transmit information on the prediction residual signal more efficiently, a Code-Excited Linear Prediction Coding (CELP) System and a Multi-Pulse Excitation Coding (MPC) System have been proposed. In the Code-Excited Linear Prediction Coding (CELP) System, a prediction residual vector is vector quantized, an index thereof is transmitted, and in the Multi-Pulse Excitation Coding (MPC) System, a prediction residual vector is modelled by a sequence of a limited number of pulses, and an optimum pulse position and an optimum pulse amplitude are transmitted.
However, when the above coding systems are used in situations wherein a transmission line error may occur frequently, such as mobile communication, error correcting coding or correction of a parameter containing an error, are required to prevent degradation of a signal due to the transmission line error.
In the correction of a parameter, a parameter containing an error is corrected by interpolation or extrapolation from the other parameters received at times near the time the parameter containing the error is received. However, the interpolation or extrapolation of parameters degrade a regenerated speech signal when parameters do not contain an error. Therefore, it is desirable to carry out the above operation only for the parameter containing the error.
In particular, in a speech signal coding system wherein a pitch prediction coefficient and a pitch period are obtained by long-term prediction analysis, and transmitted, the pitch period is a most important parameter for a voiced sound portion of a speech signal, and therefore, an error in the pitch period information will seriously degrade the quality of the regenerated sound.
However, since speech signals contain an unvoiced sound, which is non-periodic, the correction of an error by interpolation or extrapolation is difficult for a transmission line error in the pitch period even when the error is detected by an error detecting code in a speech signal decoding apparatus.